1. Field of the Invention
The present invention relates to a density control in an image forming apparatus.
2. Related Background Art
In color image forming apparatuses of the electrophotographic type, a full-color image is formed on the recording paper by repeating multiple times a process of transferring a toner image formed by electrification, exposure and development on a photosensitive drum onto the recording paper. An image forming apparatus of this type was described in Japanese Laid-open Patent Application No. 50-50935.
The conventional art will be described below based on the accompanying drawings.
FIG. 16 is a cross-sectional view showing an example of a color image forming apparatus. As shown in the figure, within the apparatus, there are disposed a photosensitive drum 1, a roller electrifier 3, and to the right side of the photosensitive drum 1, a plurality of developing cartridges 4a, 4b, 4c, 4d each having a toner container and a developing unit integrally formed as a cartridge, which can be exchanged if the toner contained has been consumed, and freely detached from a main unit of the apparatus, each of the developing cartridges 4a, 4b, 4c, 4d being carried by a rotatable support 9, and having a respective developing opening face 5a, 5b, 5c, 5d set on the same cylinder about the center of a rotational shaft of the support 9.
FIG. 17 is a cross-sectional view for explaining the detailed constitution of developing cartridges as shown in FIG. 16.
In the figure, a yellow toner 4a-1, a magenta toner 4b-1, a cyan toner 4c-1, and a black toner 4d-1 are contained within the developing cartridges 4a, 4b, 4c, 4d, respectively, and further the application rollers 6a, 6b, 6c, 6d and the toner regulating members 7a, 7b, 7c, 7d are provided, wherein the toner application rollers 6a, 6b, 6c, 6d are rotated with the rotation of developing rollers 8a, 8b, 8c, 8d to apply the toners onto the developing rollers 8a, 8b, 8c, 8d.
The material of the regulating members may be nylon when the toner is negatively polarized, or silicone rubber when it is positively polarized. That is, the material electrified oppositely to the polarity of the toner is preferred. Also, the peripheral speed of the developing rollers 8 is preferably chosen in a range of 1.0 to 2.0 times that of the photosensitive drum 1.
The developing cartridges 4a, 4b, 4c, 4d mounted on the rotational shaft of the support 9 are driven so that any one of the developing opening faces 5a, 5b, 5c, 5d of the developing cartridges 4a, 4b, 4c, 4d is always opposed to the surface of the photosensitive drum 1, as shown in FIG. 17. One example of the driving method was detailed in Japanese Laid-open Patent Application No. 50-93437.
Note that the developing cartridge 4 can eliminate the trouble in the toner replenishment or maintenance, and can be readily exchanged by the user having no special knowledge or technique. Also, the toner useful for the developing cartridge 4 may be preferably one component developer with which the cartridge can be made simple and low cost, and easily constructed in smaller size. However, there will be no harm in using two component developer.
In FIG. 16, to the left side of the photosensitive drum 1, a transfer drum 10 having a function of carrying the transfer sheet and transferring a toner image on the photosensitive drum 1 onto the transfer sheet carried therein is disposed.
With the above constitution, the photosensitive drum 1 is driven by driving means, not shown, in a direction of the arrow as shown at a peripheral speed of 100 mm/sec.
The photosensitive drum 1 is constituted by applying a photoconductor containing an organic photosensitive component (OPC) on the peripheral external face of an aluminum cylinder having a diameter of 40 mm, but A--Si, CdS or Se may be used instead of OPC.
Upward of the main unit of the apparatus, a laser diode 11 constituting an exposure unit, a polygon mirror 13 driven for rotation by a high speed motor 12, a lens 14 and a reflection mirror 15 are disposed, whereby the electrifying roller 3 is electrified evenly at approximately -700 V with the application of an AV voltage having an AC frequency of 700 Hz and a Vp-p (peak-to-peak) voltage of -1500 V superposed on a DC voltage of -700 V.
If a signal according to image information of magenta is input into the laser diode 11, a laser beam is radiated along an optical path 16 onto the photosensitive drum 1, so that a latent image is formed at a voltage of approximately -100 V on a portion of the photosensitive drum 1 radiated by the laser beam. Further, when the photosensitive drum 1 is rotated in a direction of the arrow, the latent image is visualized by a developing cartridge 4b.
Further, a transfer sheet is supplied from within a transfer cassette 17 by a pickup roller 18, synchronously with forming the image onto the photosensitive drum 1, and adsorbed onto the transfer roller 10. This transfer roller 10 has an elastic layer 20 having a thickness of 2 mm wrapped around a metallic cylinder 19 having a diameter of 156 mm, and further a PVDF 21 having a thickness of 100 .mu.m wrapped around an upper layer thereof, and is rotated at substantially the same speed as the photosensitive drum 1 in a direction of the arrow.
If the transfer sheet is supplied onto this transfer roller 10, the transfer sheet is held by a gripper 22, so that the toner image on the photosensitive drum 1 is transferred onto the transfer sheet with a voltage difference between the photosensitive drum 1 and the transfer roller 10.
The above process is repeated as well for each color of cyan, yellow and black to form a multi-color toner image on the transfer sheet.
The transfer sheet having the color image formed is peeled from the transfer roller 10 by a separation electrifier 2 and a separation claw 24, and further fused and fixed by a fixing unit 25 for heating and pressing which is conventionally well known to form a full-color image.
The remaining toner on the photosensitive drum 1 which has not been transferred onto the transfer sheet is cleaned off by a cleaning unit 26 comprised of a fur brush or blade means which is well known. Also, the toner adhering to the transfer roller 10 is preferably cleaned off by a fur brush or a transfer roller cleaning unit 27, as required.
By the way, the image forming apparatus as above described has the disadvantage that the image density change or the gradation reproducibility may become unstable, due to variations in the use environment, or in particular, humidity variations in the environment.
One of the causes of this disadvantage is the dependency of the transfer characteristics on the humidity, wherein the transfer bias is necessary to vary in a range from 2000 V to 4000 V to obtain a constant transfer current. For this purpose, a density sensor 50 is provided to control the image density so that the image density change or the gradation reproducibility may not be unstable. Conventionally, two methods have been adopted to control the image density, including a first method by detecting the density of toner image formed on the photosensitive drum and a second method by detecting the density of transfer image formed on the transfer drum.
Next, the constitution of the density sensor 50 will be described in detail.
FIG. 18 is a block diagram for explaining the configuration of a density control circuit using the density sensor 50 as shown in FIG. 16.
In the figure, 50 is the density sensor, 51 is an LED light source, 52a is a light receiving unit of the reflected light from the transfer drum 10, and 52b is a light receiving unit placed at a location capable of directly receiving a part of light from the LED light source 51. A light beam emitted from the LED light source 51 is radiated on to the toner image on the transfer drum, with a part of the light directly entering the light receiving unit 52b.
On the other hand, a light beam radiated on to the toner image on the transfer drum is reflected (absorbed) in proportion to the density of the toner image, the reflected light arriving at the light receiving unit 52a.
100 is a sequence control substrate, 101 is a CPU, 53 is a light source light amount control unit for controlling the LED light source 51 in accordance with a signal from the CPU 101, 54 to 56 are A/D converters, 57 is a monochrome data process unit, and 58 is a color data process unit.
With the above constitution, a light beam from the LED light source 51 which has arrived at the light receiving unit 52b is converted into the digital signal by an A/D converter 54 and sent to the CPU 101. The light emitted from the LED light source 51 and reflected from the transfer drum, upon arriving at the light receiving unit 52a, is converted into the digital signal by an A/D converter 55, when the toner image on the transfer drum is black (K), and passed to the monochrome data process unit 57, whereby by its comparison with a signal from the light receiving unit 52b, the CPU 101 sends an LED control signal to the light source light amount control unit 53 to adjust the light amount of the LED light source 51.
In a region from about 800 to 1000 nm which is a light emitting wavelength region of the LED, the black toner will absorb the light, while the color toners (M, Y, C) will reflect the light. Accordingly, when the density of color toner image is measured, the light amount of the LED light source 51 can be similarly adjusted by its comparison with the signal from the light receiving unit 52b through the color data process unit 58.
However, in the conventional example as above described, the light amount emitted from the light source results in the detection light amount being about 1/64, when the density detection is attempted for an image having a density of 1.8, for example.
That is, considering that the signal voltage of 5 V is a maximum detection value, the light amount is reduced to a level as low as about 78 mV, and likely affected by the noise.
As shown in FIG. 18, the density sensor unit is disposed in proximity to the surface of the transfer drum, with a measuring point of the density sensor unit located immediately behind a current transfer position from the photosensitive drum, so that the density sensor is placed remotely from the sequence control substrate, and likely affected by the noise.
In the single color printer, the output image quality could be compensated, but in the full-color printer, the image forming condition of each color component constituting the designated color was required to precisely set, even though the density was stabilized by enhancing the accuracy of each color component.
Also, there was a drawback that the parameters need be switched intricately at every time of use, because the parameters once set may be significantly affected due to environmental variations (e.g., atmospheric temperature, humidity, atmospheric pressure, sensor dirt, etc.).